Influenza virus infection causes between three and five million cases of severe illness and between 250,000 and 500,000 deaths every year around the world. In the United States alone, 5% to 20% of the population becomes infected with influenza virus each year, with the majority of these infections caused by the influenza A virus. (See, e.g., Dushoff et al., (2006) Am J Epidemiology 163:181-187; Thompson et al., (2004) JAMA 292:1333-1340; Thompson et al., (2003) JAMA 289:179-186.) Approximately 200,000 people in the United States become hospitalized with influenza-related complications every year, resulting in 7,000 to 30,000 deaths annually. The burden associated with influenza virus infection on health care costs and lost productivity is extensive. Hospitalization and deaths mainly occur in high-risk groups, such as the elderly, children, and chronically ill.
Influenza viruses are segmented membrane-enveloped negative-strand RNA viruses belonging to the Orthomyxoviridae family. Influenza A virus consists of 9 structural proteins and 1 non-structural protein, which include three virus surface proteins: hemagglutinin (HA or H), neuraminidase (NA or N), and matrix protein 2 (M2). The segmented nature of the influenza viral genome allows the mechanism of genetic reassortment (i.e., exchange of genome segments) to take place during mixed infection of a cell with different influenza viral strains. Annual epidemics of influenza occur when the antigenic properties of the viral surface proteins hemagglutinin and neuraminidase are altered. The mechanism of altered antigenicity is twofold: antigenic shift, caused by genetic rearrangement between human and animal viruses after co-infection of host cells with at least two viral subtypes, which can cause a pandemic; and antigenic drift, caused by small changes in the hemagglutinin and neuraminidase proteins on the virus surface, which can cause influenza epidemics.
Influenza A viruses may be further classified into various subtypes depending on the different hemagglutinin and neuraminidase viral proteins displayed on their surface. Each influenza A virus subtype is identified by the combination of its hemagglutinin and neuraminidase proteins. There are 16 known HA subtypes (H1-H16) and 9 known NA subtypes (N1-N9). The 16 hemagglutinin subtypes are further classified into two phylogenetic groups: Group1 includes hemagglutinin H1, H2, H5, H6, H8, H9, H11, H12, H13, and H16 subtypes; Group2 includes hemagglutinin H3, H4, H7, H10, H14, and H15 subtypes.
Hemagglutinin promotes viral attachment and entry into the host cell; neuraminidase is required for viral budding from the infected cell. The hemagglutinin of influenza A virus comprises two structurally distinct regions—a globular head region and a stalk or stem region. The globular head region contains a receptor binding site which is responsible for virus attachment to a target cell. The stalk (or stem) region of hemagglutinin contains a fusion peptide which is necessary for membrane fusion between the viral envelope and an endosomal membrane of the infected cell. (See, e.g., Bouvier and Palese (2008) Vaccine 26 Suppl 4: D49-53; Wiley et al., (1987) Ann Rev Biochem 556:365-394.)
Current treatment for influenza virus infection includes neuraminidase inhibitors, such as oseltamivir and zanamivir. Oseltamivir is a widely used prophylactic and early therapeutic treatment option for influenza A virus infection. (See, e.g., Kandel and Hartshorn (2001) BioDrugs: Clinical Immunotherapy, Biopharmaceuticals and Gene Therapy 15:303-323; Nicholson et al., (2000) Lancet 355:1845-1850; Treanor et al., (2000) JAMA 283:1016-1024; and Welliver et al., (2001) JAMA 285:748-754.) However, oseltamivir treatment must begin within 48 hours of symptom onset to provide a significant clinical benefit. (See, e.g., Aoki et al (2003) J Antimicrobial Chemotherapy 51:123-129.) This liability compromises oseltamivir's ability to treat severely ill patients, who are typically beyond the optimal 48-hour treatment window at the time of seeking treatment. Therefore, significant focus has recently been placed on identifying influenza virus therapeutics to treat hospitalized influenza virus infected patients. One strategy has focused on development of human monoclonal antibodies (mAbs) that target a highly conserved epitope on the stalk of influenza A virus hemagglutinin. (See, e.g., Corti et al., (2011) Science 333:850-856; Ekiert et al., (2009) Science 324:246-251; Ekiert et al., (2011) Science 333:843-850; Sui et al., (2009) Nature Structural & Molecular Biology 16:265-273; Dreyfus et al., (2012) Science 337:1343-1348; Hu et al., (2013) Virology 435:320-328; Clementi et al., (2011) PLoS One 6:1-10. See also International Patent Application Publication Nos: WO2009/115972, WO2011/117848, WO2008/110937, WO2010/010466, WO2008/028946, WO2010/130636, WO2012/021786, WO2010/073647, WO2011/160083, WO2011/111966, W02002/46235, and WO2009/053604; U.S. Pat. Nos. 5,631,350 and 5,589,174.)
Several reports have described monoclonal antibodies (mAb) that bind hemagglutinin and broadly neutralize influenza A virus. For example, Corti et al. (supra) described antibody FI6v3, which was cloned from a human plasma cell and shown to neutralize human influenza A viruses belonging to both Group1 and Group2 hemagglutinin subtypes. The FI6v3 mAb was discovered as a result of a heroic effort of analyzing approximately 104,000 human plasma cells. Additionally, Dreyfus et al. (supra) recently described the identification of antibody CR9114 by phage display panning; antibody CR9114 was shown to bind to a highly conserved stalk epitope shared between influenza A virus and influenza B virus hemagglutinin.
Despite these reports, a need still exists in the art for novel influenza A virus therapies effective against Group1 and Group2 influenza A virus subtypes. The present invention meets this need and provides other benefits for the treatment of influenza A virus infection.